After any meeting, success or comment, it is never bad to track the keys. It shows that you respect your customers and even if your product did not match you this time, the customer knows you are a polite and professional college, it's like doing business again.
Email or snail mail? You know best; think about corporate culture of customers. Usually, email is fine, especially if you are thanking you for something informal like time.
Your tone is also the only one you can rate. When doubt is more formal better. You do not have to be cold or content but be respectful.
Here are some examples that you can customize to your own situation.
It was a pleasure to meet you yesterday. Your ideas about new methods of sale in our area were intuitive and good help to me. Thank you for making time to meet me and share your thoughts.
—————————————–  Dear Dr. Price,
Thank you for seeing me yesterday. I appreciate the opportunity to discuss new products with you. Your views are always helpful and interesting. I hope the latest products fit your needs.
It's always a pleasure to see you!
It was a pleasure to finally get a chance to meet you today. Your research project sounds very interesting and offers a summary value for mobile operators. As has been the case, my team has been doing a great job in making the data collection that your research requires.
Please let me know if I can provide you with more information about how we want to work on the project.
Looking forward to talking to you soon.
Thank you for your time today. Jack and I thank you very much for your input into our new business development strategy. I know you have many requirements on your schedule now, so you've taken as much time as you did to guide us, even more welcome.
We will be sure to call you again when the schedule is complete so that you can provide your confirmation tag and all final comments.